Check it on a flat level surface, tires inflated to running pressures. I'd measuer both from ground to frame and ground to body in several areas (to the frame horns and the fender lips are good places). Look for any variation side to side. If it's in the frame, then there's a spring issue, if it's in the body, then perhaps something's mis-adjusted or perhaps you have a weak body mount. Is it just in one part of the body? Or the whole truck?
